An art has been known for distributing a difference between map data of a version, in which map elements, for example, roads are not updated, and map data of a version, in which the map elements have been updated, as map difference data items to a vehicle navigation apparatus. A vehicle navigation apparatus, which has received the distributed map difference data items, updates the map data with the map difference data items.
For example, patent literature 1 has disclosed an art that distributes, to a vehicle navigation apparatus, map difference data items created with a map element as a unit instead of map difference data items created with a division as a unit. Herein, division is a segment into which each of layers of map data is divided. According to the technology disclosed in the patent literature 1, for map elements which have dependent relationship and to be updated, the updates of map elements in different versions of map data are traced back and the map elements are grouped in the traced back version of the map data to generate the map difference data. This configuration tries to prevent occurrence of incompatibility in a road network. What is referred to as the dependent relationship is a relationship causing a certain map element to become incompatible with other map elements unless other map elements are updated beforehand in case the certain map element is updated.
However, in the technology disclosed in patent literature 1, a communication traffic volume for distribution of map different data items may get too large. The following will describe this difficulty in detail.
After one distribution of map difference data items to a vehicle navigation apparatus and before the next distribution of the map difference data items, when multiple versions of map data items need to be updated, map difference data items including all updates of map elements that have a dependent relationship in the multiple versions need to be distributed. The larger the number of versions that have to be traced is, the larger the number of updates of map elements having the dependent relationship is. Thus, a communication traffic volume for distribution of map difference data items may get extremely large.
In case a restriction is imposed on a communication traffic volume for one distribution or on a communication traffic volume during one day or one month, when the communication traffic volume of map difference data items gets too large, the distribution of the map difference data items may be failed. Further, assume that the map difference data items are simply divided into groups so that part of the groups can be distributed under a restricted communication traffic volume. In this case, since only part of the divided map difference items is employed, updated map elements in a road network or the like become incompatible with other map elements.
The map difference data items may also be divided into smaller groups for the distribution under a predetermined communication traffic restriction. When the map difference data items are simply divided into smaller groups and distributed by multiple times, map difference data that is less necessitated by a user may be distributed prior to the map difference data that is more necessitated by the user at a higher frequency. Accordingly, update of a map element that is less necessitated by a user is performed prior to update of a map element that is more necessitated at a higher frequency.